In order to solve the problem that the large amount of mud generated during the construction of the clay soil layer restricts the efficient and green construction of the project, the waste mud of the construction of a certain tunnel was selected, and anionic polyacrylamide (APAM), non-ionic polyacrylamide (NPAM) and polyaluminum chloride (PAC) were added respectively to carry out laboratory flocculation sedimentation test. The best flocculant and its optimal addition amount were obtained by combining with the flocculating mechanism of flocculant, and the field flocculation test was carried out. The results show that 0.5% APAM has good flocculation effect, which can quickly achieve the ideal flocculation, sedimentation, floating and sludge dewatering.